The Fly's Eye array operated out of Dugway Proving Ground,
military base in the desert of western Utah, from 1981 to 1993;
it pioneered the
"air fluorescence technique"
the energies and directions
of ultrahigh-energy cosmic rays
based on faint light emitted
by nitrogen air molecules
cosmic-ray air shower traverses the atmosphere.
In 1991, the
Fly's Eye detected a cosmic ray
that still holds the world
record for highest-energy particle.
University of Utah
Where had the
Oh-My-God particle come from? How could it
possibly exist? Did it really?
The questions motivated
astrophysicists to build bigger, more sophisticated detectors
that have since recorded hundreds of thousands more "ultrahigh-energy cosmic rays" with energies above 1 EeV,
including a few hundred "trans-GZK" events above the 60 EeV
cutoff (though none reaching 320 EeV).
In breaking the
limit, these particles challenged one of the farthest-reaching
predictions ever made.
It seemed possible that they could offer
a window into the laws of physics at otherwise unreachable
scales - maybe even connecting particle physics with the
evolution of the cosmos as a whole.
At the very least, they
promised to reveal the workings of extraordinary astrophysical
objects that had only ever been twinkles in telescope lenses.
But over the years, as the particles swept brushstrokes of light
across sensors in every direction, instead of painting a
telltale pattern that could be matched to, say, the locations of supermassive
black holes or colliding galaxies, they created
"It's hard to explain the cosmic-ray data with any
particular theory," said
Paul Sommers, a semiretired astrophysicist at Pennsylvania
State University who specializes in ultrahigh-energy cosmic
problems with anything you propose."
Only recently, with the discovery of a
cosmic ray "hotspot" in the sky, the detection of related
high-energy cosmic particles, and a better understanding of
physics at more familiar energies, have researchers secured the
first footholds in the quest to understand ultrahigh-energy
"We're learning things very rapidly," said
Tim Linden, a theoretical astrophysicist at the University
A logarithmic plot showing the flux of cosmic rays
function of energy.
The line has two bends (where its slope
known as the cosmic-ray energy spectrum's
Olena Shmahalo/Quanta Magazine.
Original data via S. Swordy, U. Chicago.
Thousands of cosmic rays bombard each square foot of Earth's
atmosphere every second, and yet they managed to elude discovery
until a series of daring hot-air-balloon rides in the early
As the Austrian physicist
Victor Hess ascended miles into
the atmosphere, he observed that the amount of ionizing
radiation increased with altitude. Hess measured this buzz of
electrically charged particles even during a solar eclipse,
establishing that much of it came from beyond the sun.
received a Nobel Prize in physics for his efforts in 1936.
Cosmic rays, as they became known, arc through Earth's
magnetic field from every direction, and with a smooth spread of
energies. (At sea level, we experience the low-energy, secondary
radiation produced as the cosmic rays crash through the
Most cosmic rays are single protons, the positively
charged building blocks of atomic nuclei; most of the rest are
heavier nuclei, and a few are electrons. The more energetic a
cosmic ray is, the rarer it is.
The rarest of all, those that
are labeled "ultrahigh-energy" and exceed 1 EeV, strike each
square kilometer of the planet only once per century.
Plotting the number of cosmic rays that sprinkle detectors
according to their energies produces a downward-sloping line
with two bends - the energy spectrum's "knee" and "ankle." These
seem to mark transitions to different types of cosmic rays or
progressively larger and more powerful sources.
The question is,
which types, and which sources?
Like many experts,
Karl-Heinz Kampert, a professor of astrophysics at the
University of Wuppertal in Germany and spokesperson for the
Pierre Auger Observatory, the world's largest ultrahigh-energy
cosmic ray detector, believes cosmic rays are accelerated by
something like the sonic booms from supersonic jets, but on
Shock acceleration, as it's called,
fundamental process which you find on any scale in the
Kampert said, from solar flares to star explosions
(supernovas) to rapidly spinning stars called pulsars to the
enormous lobes emanating from mysterious, super-bright galaxies
known as active galactic nuclei.
All are cases of heated matter
(or "plasma") flowing faster than the speed of sound, producing
an expanding shock wave that accumulates a crust of protons and
The particles reflect back and forth across the
shock wave, trapped between the magnetic field of the plasma and
the vacuum of empty space like little balls ping-ponging between
table and paddle.
A particle gains energy with every bounce.
will escape," Kampert said, "and move through the universe
and be detected by an experiment."
Cosmic rays are most likely energized through
reflecting back and forth across a shock
wave that is produced
when plasma flows faster than the speed of
The stronger and larger the magnetic field of the plasma,
the more energy it can impart to a particle.
cosmic rays surpass
1 exa-electron volt (EeV).
Trying to match different shock waves to parts of the
cosmic-ray energy spectrum puts astrophysicists on shaky ground,
They would expect the knee and ankle to mark the
highest points to which protons and heavier nuclei
(respectively) can be energized in the shock waves of supernovas
- the most powerful accelerators in our galaxy.
suggest the protons should max out around 0.001 EeV, and indeed,
this aligns with the knee.
Heavier nuclei from supernova shock
waves are thought to be capable of reaching 0.1 EeV, making this
number the expected transition point to more powerful sources of
"extragalactic" cosmic rays.
These would be shock waves from
singular objects that aren't found in the Milky Way or in most
other galaxies, and which could well be galaxy-size themselves.
measured ankle of the spectrum,
"the only place where it looks
like there's a clear transition," Sommers said,
...lies around 5 EeV, an order of magnitude past the theoretical
maximum for galactic cosmic rays.
No one is sure what to make of
Past the ankle, at around 60 EeV, the line dips toward zero,
forming a sort of toe. This is probably the GZK cutoff,
the point beyond which cosmic rays can only tarry for so long
before losing energy to ambient cosmic microwaves generated by a
phase transition in the early universe.
The existence of the
cutoff, which Kampert calls "the only firm prediction ever made"
about cosmic rays, was
established in 2007 by the Fly's Eye's successor - the High
Resolution Fly's Eye experiment, or HiRes.
From there, the
energy spectrum reduces to a trickle of trans-GZK cosmic rays,
finally ending, at 320 EeV, with a single data point: the
The presence of the GZK cutoff means that the laws of physics
are operating as expected. Rather than disproving those laws,
trans-GZK cosmic rays probably do originate nearby (reaching
Earth before ambient microwaves sap their energy).
For a maddening 20 years, the particles appeared to
come from everywhere and nowhere in particular. But finally a
hotspot has developed in the Northern Hemisphere.
Could this be
the invisible gorilla hurtling bowling balls toward Earth?
In Utah, a three-hour drive from the site of the original
Fly's Eye, its latest descendant sprawls across the desert:
762 square-kilometer grid of detectors called the Telescope
The experiment has been tracking the
multi-billion-particle "air showers" produced by
ultrahigh-energy cosmic rays since 2008.
"We've been watching
the hotspot increase in statistical significance for several
Gordon Thomson, a professor of physics and astronomy at the
University of Utah and spokesperson for the Telescope Array.
Of the 87 cosmic rays surpassing 57 EeV
detected thus far
by the Telescope Array,
27 percent come from 6 percent of
The hotspot centers on the constellation Ursa
Tokyo Institute for Cosmic Ray Research
The hotspot of trans-GZK cosmic rays, which centers on
constellation Ursa Major, was initially too weak to be
But in the past year, it has reached an
estimated statistical significance of "four sigma," giving
it a 99.994 percent chance of being real. Gordon Thomson and his
team must reach five-sigma certainty to definitively claim a
(Thomson hopes this will happen in the group's
next data analysis, due out in June.)
Already, theorists are
treating the hotspot as an anchor for their ideas.
"It's really exciting," said Linden.
With more data, he
explained, the location of the source can be pinpointed
within the hotspot (which gets smeared out by the deflection
of cosmic rays as they pass through the galaxy's and Earth's
By tracking other types of particles
coming from the same spot in the sky,
"you have a model of
how the source works over many orders of magnitude in
energy," he said.
The invisible gorilla would materialize.
Meanwhile, some of those other particles are slowly
piling up in the sensors of the
IceCube detector, a
cable-infused, cubic-kilometer block of ice buried beneath
the South Pole.
For the past four years,
monitored the rare ice tracks of neutrinos, lightweight
elementary particles that usually flit right through matter
and thus require immense efforts to detect, but which are
produced in abundance from physical processes throughout the
Every so often, cosmic neutrinos interact with atoms and
produce radiation as they pass through IceCube; their
directions of travel trace a new map of the cosmos that can
be compared to the maps of ultrahigh-energy cosmic rays and
those of light.
In 2013, IceCube scientists reported
Observation of PeV-energy Neutrinos with IceCube) the observation of the first-ever
very-high-energy neutrinos - a pair of 0.001-EeV particles
nicknamed "Bert" and "Ernie" that might have come from the
same sources that yield ultrahigh-energy cosmic rays.
Neutrinos have a big advantage over cosmic rays as
messengers from the most powerful objects in the universe:
Because they are electrically neutral, they move in straight
"Since neutrinos travel to us uninhibited from the
source, they might be able to open up a new window on the
Olga Botner of Uppsala University in Sweden, IceCube's
At the South Pole, the IceCube Neutrino Observatory
is approaching the mystery of ultrahigh-energy cosmic
by hunting related cosmic neutrinos,
with atoms every so often
while passing through the
cubic-kilometer block of ice.
Emanuel Jacobi, NSF
Of the 54 high-energy neutrinos that IceCube has detected
as of its latest analysis,
reported in early May, four originate from the vicinity
of the cosmic-ray hotspot. (Neutrinos can enter the detector
after traveling through Earth from the northern sky.)
This "hint of a correlation," as Linden described it, could be a
clue: Cosmic rays take longer to get to Earth than
neutrinos, so a common source would have to have been
pumping out energetic particles for many years.
source candidates such as gamma-ray bursts would be ruled
out in favor of stable objects - perhaps a star-forming
galaxy with a supermassive black hole at its center.
next few years we're going to get that many more neutrinos,
and we'll see how this correlation plays out," Linden said.
For now, though, the correlation is very weak.
staking my foot in the ground," he said.
Alongside cosmic rays and neutrinos, cosmic
(high-energy photons) will serve as a third messenger in the
They're the subject of several major searches
including the HESS (High Energy Stereoscopic System)
experiment in Namibia - named in honor of the father of
cosmic rays - and VERITAS (Very Energetic Radiation Imaging
Telescope Array System) in Arizona, for which David Kieda, the
former Fly's Eye scientist, now works.
combination of cosmic-ray, neutrino and gamma-ray data
should help locate and sharpen astrophysicists' picture of
the most powerful accelerators in the universe.
will organize around the hotspot.
Thomson has his money on threads of galaxies and dark
matter called "filaments" that are draped throughout the
cosmos and which, at hundreds of millions of light-years
long, are among the largest structures in existence.
a filament in the direction of the hotspot.
something in the filament," Thomson said.
In any case, he
have an idea now of interesting places to look. And all
we need to do is collect more data."
Draining the Pool
Kampert, of the
Pierre Auger Observatory, is approaching
the mystery of ultrahigh-energy cosmic rays from a different
direction, by asking:
What are they?
Victor Hess discovered cosmic rays
in a series of
in Austria between 1911 and 1913,
radiation of very high penetrating power
our atmosphere from above."
Some astrophysicists say the Auger Observatory has been
Covering 3,000 square kilometers of Argentina
grasslands, it collects far more data than the Telescope
Array, but it does not see a hotspot in the Southern
Hemisphere with anywhere near the prominence of the one in
It has detected evidence of a slight
concentration of trans-GZK cosmic rays in the sky that
overlays an active galactic nucleus called
Centaurus A as
well as another filament.
But Kampert says Auger might never
collect enough data to prove this so-called "warmspot" is
Still, the dearth of clues is a mystery in itself.
"It's a very rich data set and we don't see anything,"
said Sommers, who helped design and organize the Auger
absolutely amazing to me. Back in the 1980s I would have
bet good money that if we had the statistics we have
now, there would be obvious hotspots and patterns. It
makes me really wonder."
Kampert thinks he and his colleagues must simply get
smarter about how they look for hotspots, which are surely
there; the local region of the universe is not uniformly
blanketed by objects capable of accelerating particles to
The problem is magnetic deflection, he
said. Galactic and extragalactic magnetic fields bend
protons five to 10 degrees off-course, and they bend heavier
nuclei many times that, depending on the number of protons
Auger's analysis of its air-shower events
(which integrates cutting-edge results from particle
collisions at the Large Hadron Collider) suggests that the
highest-energy cosmic rays tend to be on the heavy side,
consisting of carbon or even iron nuclei.
"If at the highest energies we have [heavier nuclei],
then your sky is always fuzzy or smeared out," Kampert said.
"It would be like doing astronomy from the bottom of a
He and his team hope to update their experiment with the
ability to identify the composition of cosmic rays on an
This will allow them to look for
correlations between only the lightest, least deflected
"Composition is really the key to understanding
the origin of the highest-energy particles," he said.
And the shift toward heavier nuclei at the far end of the
cosmic-ray energy spectrum could be a major clue itself.
Just as supernovas accelerate protons no further than the
"knee" of the spectrum and can propel only heavier nuclei
beyond that point, so too might the most powerful
astrophysical accelerators in the universe peter out.
Scientists could be glimpsing the true edge of the
cosmic-ray spectrum: the points where protons, and then
helium, carbon and iron, max out. Measuring this falloff
will help expose how the giant accelerators work - and favor
certain candidates over others.
Theorists still struggle to imagine any of those
candidates producing the sprinkle of particles in the
200-EeV range or the Oh-My-God particle at 320 - even if
they are made of iron.
"How you get a [320 EeV] particle is
not easy from any theory," Thomson said. "But it was there.
Even that fact is called into question.
Back in the early
1990s, Sommers, who was temporarily working at the
University of Utah, helped the Fly's Eye scientists analyze
their 320-EeV signal.
But although the
"big event" (as he
calls it) was "pretty well measured by the standards of the
time," the Fly's Eye hadn't fully transitioned away from
being a "monocular" experiment, analogous to one fly's eye
rather than two (a second eye was under construction); it
lacked the precision and redundancy of later stereoscopic
Sommers said that although no serious reasons for
doubting the energy estimate are known,
must be suspicious of it now. With vastly greater
exposure, the more precise, new observatories have
failed to detect any particle of such high energy.
flux of particles at energies that high must be so low
that it would have been an incredible fluke that the
Fly's Eye detected one."
The error bars that went into calculating the
particle's energy might all have been off in the wrong
direction at the same time.
If so, it was a lucky mistake
for the field, motivating new experiments without greatly
misleading researchers, since many other trans-GZK particles
And if the
Oh-My-God particle was a mistake,
well, probably no one will ever know.